Ignorance about the relative importance of events during early epochs currently allows for a rich variety of interpretations of the QSO absorption line systems. A consensus has arisen that most progress can be made when the formation of the absorption systems is treated as an integral part of a general scenario for galaxy formation.
Among the most detailed of such descriptions is the work of Ostriker,
Ikeuchi and their co-workers.
Ikeuchi and Ostriker
(1986)
present a unified picture for the
formation of ordinary galaxies, young low mass blue compact galaxies and
Ly-
clouds.
They hypothesize the occurrence of explosions (pregalactic objects or
expanding voids)
at redshifts 5 - 20. The shocks produced by these explosions ionize the
intergalactic
medium and create shells which are unstable to fragmentation. The large mass
fragments collapse to form galaxies, while those with low masses survive
as Ly- clouds
confined by the pressure of the ambient intergalactic medium.
In this picture there is thus a natural continuity between the
Ly- clouds and the
dwarf galaxies which might account for the metal line systems.
The chief weakness of this model is that it invokes hypothetical
energetic explosions.
There is no consensus that these explosions are required to be strong enough
to ionize the intergalactic medium (e.g.
Donahue and Shull 1987).
Rees (1986)
notes that the pressure of the intergalactic medium is thus determined in an
ad hoc manner, a point which is also made by
Vishniac and Bust (1987)
who find that the clouds may
be confined by ram pressure rather than by the pressure of the ambient
intergalactic
medium. If the sites of the explosions are young galaxies, then Vishniac
and Bust
argue that one must either postulate the destruction of clouds in dense
environments, or following
Salmon and Hogan
(1986),
require that galaxy clustering be
minimal at redshifts 
2.5.
Shocks also play a role in many other models. Hogan (1987) has recently
discussed a possible origin for the
Ly- clouds in the context of
the Fall and Rees
(1985)
theory for the collapse of protogalaxies. Hogan notes that if the
Ly- clouds
are of relatively
low ionization, their contribution to the mass density of the universe
is insignificant compared to galaxies
(Tytler 1987a).
They could then arise in a very minor gas
phase occurring during the formation of galaxies. Shocks produced as
protogalaxies
are assembled from subunits would be observed as
Ly- clouds with total column
densities in excess of 1017 cm-2, with lower N(HI)
occurring in clouds which are
moderately ionized. Since the protogalaxies would have sizes of
300 kpc prior to
collapse, coherent gas flows might produce
Ly- lines which are closely
correlated in
velocity over lines of sight separated by tens of kpc, as required by
the observations
of Foltz et al. (1984).
This model avoids the need to postulate a medium to confine
the Ly- clouds, but it is then
unclear whether the redshift evolution of the clouds
will match the observations, since this will depend on the unknown
details of the evolution of protogalaxies.
Attempts to model long lived
Ly- clouds without using
pressure support center on the use of cold dark matter.
Rees (1986)
notes that low mass 'minihaloes' are
predicted to exist at the present epoch in the cold dark matter theory
for the formation
of galaxies. Such minihaloes may be able to trap and stably confine
gas. Radii of
order 8 kpc are indicated, while the gas density would be low enough to
allow a high level of ionization.
Ikeuchi and Norman
(1987)
present further details of this type of
model. In common with Rees, they stress that there need be no sharp demarcation
between the Ly- and the metal
line systems, since collapse and star formation are
inevitable in those haloes which have somewhat higher masses than the
Ly- clouds.
It will be important to test how well these models can preserve the distinction
between the Ly- and metal line
system in terms of clustering. If both types of system
come from the low mass end of the same spectrum of perturbations, then
it may be
that the clustering of the metal systems is largely due to hydrodynamic
velocities of
clouds which arise in the same events which produce the metal enrichment.
Prospects for observational discrimination between these interpretations seem
excellent. The ionization of the
Ly- clouds should be revealed by the
comparison of H
and He lines. The frequency of occurrence of low temperature
Ly- clouds may be
estimated from the numbers of lines with extremely low velocity dispersions.
Observations of low redshift systems will be especially exciting. Here
we can see whether
metal or Ly- systems are
associated with dwarf galaxies, we can search for the
expected changes in clustering, and extend constraints on the evolution
of the clouds up to about 10 billion years.
It is appropriate to end with an analogy drawn from the recent history of
extragalactic astronomy. Following their discovery, QSOs were widely
regarded as a
totally unique new phenomena. We are now aware that in spite of great
differences
in luminosity, appearance, and space density, the Seyfert galaxies
display much of
the same physics. A great deal has been learnt from this recognition of
underlying
similarity. It is suggested that a similar situation applies to the
Ly- and the metal
line systems seen in QSO spectra.